This invention refers to plasma structuring of a substrate surface.
To be more specific, this invention relates to a structuring device for processing a surface of a substrate, comprising a substrate chamber for mounting the substrate and a reaction chamber enabling a gas reaction at a given operating pressure, the reaction chamber having at least one gas inlet for a reaction gas and at least one injection outlet leading into the substrate chamber.
A plasma etching device of this type was proposed by Larson et al. in J. Vac. Sci. Technol. B, Vol. 10 (1992), p. 27. In this device, a glass tube widens to an expansion serving as plasma chamber, tapers back and then performs a V-shaped bend. The etchant gases enter the etching device by way of the inlet of the tube, flow through the larger plasma chamber where a radio-frequency plasma discharge is maintained, continue downstream past the vertex of the V, and are pumped off through the outlet of the tube. At the vertex an aperture is made in the thinned tubing wall. Below the aperture the substrate is positioned on a base which, in the embodiment used by Larson et al., is the upper face of a waveguide used for monitoring and which, together with the tubing wall around the aperture and vacuum-sealed by means of an O-ring, forms an etching chamber enclosing the substrate. By this etching method, it is possible to achieve a spatially confined etch process as defined by the shape of the perimeter of the etching chamber. However, this approach does not allow for high-resolution etching or simultaneous imaging during the etch. The shaping of the etching chamber defines the smallest size of the features which can be fabricated using this method. Since the size of the chamber cannot be easily made small, this method is not well suited for making structures in the micrometer and submicrometer-size range.
Structuring of features in the micrometer range and below, so-called microfabrication, is used in various fields of technology such as electronics, sensorics, integrated optics, micro-actuators, and micro-acoustic devices. Structuring here means the creating of a desired spatial pattern on the substrate surface, and comprises material processing such as removing, e.g. etching; growing, e.g. deposition; and other ways of treatment of material, e.g. doping or chemical transformation.
With regard to etching at present time, dry etching in a weakly ionized high-density plasma is most commonly used; plasma methods are also applicable for deposition processes. The benefit of plasma etching is that it allows for anisotropy, i.e. a high directionality where the material is removed preferentially along a certain crystallographic direction on the substrate. The material to be etched is chemically converted into a volatile phase which is pumped off. It is to be noted that by changing the gas composition, it is possible to switch from etching to deposition of new material straightaway.
At present, commercial microfabrication requires a combination of different methods for deposition of photo-resist, writing the required micro-patterns in the resist, and transferring the pattern from the resist into the substrate by etching of the resist and subsequent processing of the substrate. Lithographic methods are, for instance, discussed in xe2x80x98semiconductor Fabtechxe2x80x99, ed. M. J. Osborne, 7th edition, 1998, ICG Publishing Ltd., London. The lithographic methods are indirect in a sense that the desired structures are first defined in a layer deposited only temporarily and then transferred from this layer to the substrate. Therefore, microfabrication by lithographic procedure involves a plurality of steps making the complete process complex and expensive. Features as small as 200 nm can be fabricated using optical lithography, and structures with features down to 100 nm can be fabricated using particle-beam or X-ray lithography. As the size of the features continues to shrink, the equipment required for microfabrication rapidly becomes more and more expensive with respect to cost of ownership and maintenance of the complex line.
Alternative methods to lithography use direct structuring of the substrate. One direct etching arrangement for high-resolution microfabrication is the focused ion beam (FIB) technique which allows microfabrication by vector scan of the ion beam. The FIB technique, however, is very cost intensive. Also known is the combination of FIB microfabrication and in-situ analysis by scanning electron microcopy (SEM). This combination technology proved highly valuable for failure analysis and prototype development; however, its high cost and complexity are prohibitive for most commercial applications. Instead of ions, electrons can also be used for electron-beam structuring methods. It is to be noted that these structuring methods involve using a particle-beam rather than a plasma gas. One well-known complication of using particle beams is defects introduced into the substrate by the impact and/or implantation of high-energy particles.
Also known is a flow injection system for delivering a liquid etchant onto a surface and monitoring the chemical processes by means of scanning tunneling microscopy using the delivering syringe as scanning probe, as disclosed by Noll et al., in Rev. Sci. Instrum. Vol. 66 (1995), p. 4150. This system, however, relates to the investigation of chemical and electrochemical processes; due to the surface tension, the etchant liquid will spread over a large area on the substrate and prevent successful manufacturing of small-size structures.
Another method of structuring, called xe2x80x9chot-jet etchingxe2x80x9d is described by Geis et al., in J. Vac. Sci. Technol. B 4, 315-317 and J. Vac. Sci. Technol. B 5, 363-365. There, a stream of reactive gas is produced by means of a heated jet nozzle and directed toward a substrate. The reactive gas species of the stream are generated by thermal dissociation. The distance between the nozzle outlet and the substrate is chosen so that the entire substrate area is covered by the gas stream. In the setup of Geis et al., this distance is approximately 10 cm, which corresponds to the dimension of the wafer used as substrate. In order to achieve a structuring of the etching process, Geis et al. mainly use photoresist masks, but also propose to employ stencil masks positioned above the surface of the substrate to be processed. Thus, the production of the reactive gas stream and determination of the etch pattern are determined by separate devices, and thus the hot-jet etching method employs indirect structuring as well, like indirect lithography. Therefore, it requires pre-structuring of the mask layer or the stencil mask and brings about the disadvantages as discussed above.
It is an aim of this invention to develop a novel, inexpensive technology for direct microfabrication by etching of the substrate surface in the micrometer range and below, based on direct structuring of surfaces using plasma. As a further aim, the technology should allow for local growth and chemical modifying of material on the surfaces. Moreover, structuring shall be possible without introducing damages from the particles applied. It is a further aim of the invention to allow for simultaneous microfabrication and imaging.
These aims are achieved by a structuring device as mentioned at the beginning wherein (see, e.g., FIG. 11) the substrate chamber VC is provided with a pumping system PP for maintaining a vacuum within the substrate chamber at a pressure not above the operating pressure of the gas reaction in the reaction chamber GC, the injection outlet JL is provided with at least one injection pipe ending into an injection opening OP of given width d1, the injection pipe having a length s1 not smaller than the width d1 of the injection opening, the injection pipe forming the gas particles originating from the gas reaction into a gas jet streaming out of the injection opening OP, and the injection outlet JL and/or the substrate SB are provided with a positioning means NP,SP for positioning the injection opening with respect to the substrate surface at a height of the order of or below the width of the opening as measured along the axis of the injection pipe.
This solution offers a simple and low-cost way to perform direct microstructuring. The injection opening forms the gas jet and thus directly determines the size and shape of the area etched or deposited to at any time, thus determining the size of the smallest features which can be fabricated using this method. Only the area directly in the vicinity of the opening is being processed, whereas the other areas of the sample are not affected. Another advantage is the low landing energy of the reactive particles at the substrate surface, determined by the thermal energy of the particles, i.e. in the order of 0.1 eV, limiting the detrimental effects to the surface and thus avoiding damage at the substrate. Whenever reference is made to the width of the injection opening or injection pipe, this refers the cross-sectional dimension of the opening or pipe.
The possibility of combining in one device both removing and depositing of material, by changing the composition of the gas used, offers a wide field of applications. They range from the production of integrated elements and micro-devices to the repair of surfaces or membranes such as stencil masks used in lithography, including the removal of particles or other contaminations and growth of bridges. Substrates which can be processed by structuring according to the invention are inorganic materials, e.g. metals such as aluminum or tungsten, semiconductors such as silicon; organic materials, e.g. polymers; and combinations of these materials, e.g. composite materials and integrated circuit devices. Especially if processing small structures with composition varying spatially the combination of surface treatment and imaging proves to be particularly helpful. One example of surface structuring of organic or semiconductor materials is the fabrication of molds in imprint patterns used e.g. in the production of CD disks, by means of mechanical nano-imprint technology as discussed by Chou et al., in Appl. Phys. Lett. Vol. 67 (1995), p. 3114. Another possible application is the manipulation of biological targets, such as single cells.
A preferred embodiment of the invention presented comprises a shield means AS positioned at the injection outlet and at least one injection pipe traversing the shield means. This facilitates production, replacement and maintenance of the device bearing the injection pipe. The shield means can be, for instance a patterned membrane mounted at the opening of the injection outlet.
The shield means can be positioned at a predefined distance from the substrate. Alternatively, in structuring setups where it ;s compatible with the structuring requirements, it may be favorable if the shield means is positionable by means of the positioning means at contact with the substrate.
In a preferred embodiment of the structuring device according to the invention (see, e.g., FIG. 1), which is in particular suitable for the microstructuring purposes described above and in which the size and shape of the injection opening are imaged by the gas jet onto the substrate to be structured, making it possible to process a delimited area of the substrate only, the at least one injection outlet leads through an injection opening OP into the substrate chamber VC, the injection opening OP is positioned at the end of an injection pipe having a length s1 greater than the diameter d1 of the injection opening OP, the injection pipe forming the gas particles originating from the gas reaction into a gas jet streaming out of the injection opening OP, and the injection outlet and/or the substrate are provided with a positioning means NP,SP for controlling the distance between the injection opening and the substrate surface at a height of the order of or below the diameter d1 of the opening as measured along the axis of the injection pipe.
It is further advantageous when the injection outlet or the shield means comprises at least one projecting nozzle NZ through which the at least one injection opening OP emerges. This ensures that there is sufficient space below the shield means so as to allow the volatile products of the processing of the substrate to escape freely in order to be pumped off. Moreover, this simplifies the lateral positioning of the injection opening with respect to the substrate. In particular, the injection outlet may be a tube NT terminating in a tapering end forming a nozzle NZ through which the injection opening emerges.
In order to sustain a plasma discharge in the tube or a given part of the tube, the injection outlet, in particular the projecting nozzle just mentioned, can additionally be provided with at least one electrode, e.g. two or four electrodes, for producing an electro-magnetic field inside the tube.
Furthermore, the injection outlet may comprise at least one escape opening leading into the substrate chamber away from the substrate. This makes it possible to run a continuous gas discharge, which can potentially give rise to a high gas flux, in order to maintain the discharge process in the reaction chamber, even if the required etchant gas flux through the injection pipe is very low, or if with varying injection geometries the amount of gas flux needed varies.
Furthermore it is advantageous if the length s1 of the injection pipe is greater than the injection opening width d1 by a ratio of at least 5:1 (see, e.g., FIGS. 2 and 12a). This allows creating a well-defined area of processing on the substrate for a distance between the injection opening and the substrate surface as large as the width of the opening.
Preferably the reaction chamber is a plasma reaction chamber equipped for sustaining a plasma discharge. Using a plasma reaction allows for the implementation of a wide array of processes for structuring applications. Advantageously, if e.g. surface processing is desired using a short-lived gas species or species which are difficult to store in gaseous form, these gas species can be produced in the course of a gas reaction in the reaction chamber, and thus the gas jet contains chemically active particles produced by the gas or plasma reaction from the at least one reaction gas.
In another preferred embodiment of the invention, the injection outlet and/or the substrate may be provided with a scanning means for scanning the injection opening relative to the substrate over the substrate surface. This makes a sequential microfabrication process possible, in which the substrate surface is being processed area by area, and therefore allows for imaging or writing of structures over an area of the substrate which is far greater than the width of the injection opening. It also allows for imaging the sample surface using the injection opening. The scanning means can be a device separate from the positioning means for control of the distance between the injection opening and the substrate, but advantageously these two functions will be integrated into one position control assembly.
Advantageously the positioning means is realized as a surface scanning microscope, e.g. atomic-force or shear-force scanning microscope, using the injection opening (or a nozzle) as a scanning probe. This provides for the simultaneous microfabrication and imaging with high resolution, thus yielding a highly efficient way to control processing, like for instance for in-situ analysis of the cross-sections of microdevices.
For an efficient operation of the emitted gas jet and removal of the products of structuring, e.g. etching products, the pumping system favorably maintains the vacuum in the substrate chamber at a pressure below the operating pressure of the gas reaction in the reaction chamber by at least one order of magnitude.
In another embodiment of the invention the gas reaction is pulsed. By using a non-continuous reactive gas stream, it is possible to modulate the surface processing, e.g. for short-lived species or for products of extremely low volatility.
A pulsed production of reactive gas also offers the possibility to use the processing pauses for other processing operations, such as positioning and imaging of the sample. This variant is especially useful when a nozzle is used as a probe of a scanning probe microscope.
In a further variant to the invention, the shape and/or the number of the injection opening(s) is formed according to the desired structure(s) to be formed. This allows for printing of structures onto the substrate, thus giving a reduction in processing time.
In another variant of the invention the reaction chamber is located inside the substrate chamber. This may lead to a simpler set-up of the structuring device, depending on the types of the gases and the gas reaction used, and ensures that, e.g. in the case of a rupture of the injection outlet, the reactive species produced in the reaction chamber can only arrive at the vacuum chamber, whence they are pumped off and disposed safely.
For applications where it is favorable that only a small portion of the substrate is exposed to vacuum conditions, the substrate chamber of the structuring device may be provided with an opening adapted to be covered by the substrate being mounted outside the substrate chamber. Thus only the region of the substrate covering the opening is exposed to the vacuum of the substrate chamber. For ensuring free movement of the substrate under the opening, advantageously the opening of the substrate chamber is provided with a positioning means to position the substrate to maintain a predetermined gap width between the substrate surface and said opening, and said opening is further provided with at least one surrounding sleeve provided for being pumped off to a pressure intermediate between the pressure within the substrate chamber and the ambient pressure surrounding the substrate.
For producing a nozzle for the structuring device according to the invention, one suitable method starts from a doped monocrystalline silicon wafer and comprises the following steps (see FIG. 19):
(a) etching of at least one defined window in the front side of said wafer to predetermined crystallographic planes, forming at least one pyramidal hole or elevation PH,PY,
(b) generating a stop layer DP of predetermined thickness on the front side of said wafer by diffusion using a dopant of opposite doping type with respect to the bulk of said wafer,
(c) electrochemical wet etching of the back side of said wafer to said stop layer,
(d) forming an aperture through the tip of the pyramidal hole or elevation.